1. Field of the Invention
This invention relates to perpendicular magnetic recording media mounted in various magnetic recording devices. More specifically, this invention relates to perpendicular magnetic recording media mounted in hard disk drives (HDDs) used as storage devices in computers, audio and video equipment, and similar.
2. Description of the Related Art
In recent years there has been a rapid increase in the recording density of hard disk drives (HDDs), and this trend is expected to continue. However, when using conventional magnetic recording media employing in-plane magnetic recording, because of the problem of the “thermal fluctuation” phenomenon due to which recorded signals cannot be held with stability, a recording density limit was being approached. Hence in order to address demands for substantial increases in recording densities, much research had been performed on perpendicular magnetic recording media, adopting the perpendicular magnetic recording method with its feature, diametrically opposed to the behavior of the in-plane magnetic recording method, by which bit stability increases as recording densities are increased; and perpendicular magnetic recording media are now being commercialized.
In order to raise magnetic recording medium densities, it is necessary to promote the magnetic separation of crystal grains forming the magnetic recording layer and to diminish the unit of magnetization reversal. Thermal stability, which represents the durability of a magnetic body to thermal fluctuations, is indicated by the index KuVa which is the product of the uniaxial anisotropy constant Ku and the activation volume Va. Here, Va is known to be correlated with the volume V of the magnetization reversal unit. That is, the smaller is the value of KuVa or of KuV, the lower is the thermal stability of the magnetic recording media. As is clear from this index, in order to raise the recording density, the magnetization reversal unit is reduced, and thermal stability is also diminished, so that thermal fluctuations pose a problem even for perpendicular magnetic recording media. Hence in order to maintain thermal stability even when the magnetization reversal unit is reduced, Ku must be increased.
On the other hand, it is known that the magnetic field intensity necessary during recording in an HDD is substantially proportional to the value of Ku. Hence, when Ku is increased in order to maintain thermal stability, the magnetic field intensity necessary during recording increases, and when this increase is substantial, cases occur in which recording is not possible.
Further, as the magnetization reversal unit is reduced, the demagnetizing field also grows smaller, and so the reversal magnetic field of the magnetic recording layer increases. That is, the smaller is the magnetization reversal unit, the higher is the magnetic field intensity necessary for recording.
Hence, in the interest of higher recording densities, finer magnetization reversal units and increased Ku contribute to the improvement of the recording resolution and thermal stability of the magnetic recording media, both lead to a reduced ability to record on the magnetic recording media (hereafter also expressed as “ease of recording”).
In light of the above circumstances, a method is sought to improve the thermal stability and electromagnetic transducing characteristics of magnetic recording media, without detracting from the ease of recording.
As a method of resolving this problem, in Japanese Patent Application Laid-open No. 2006-48900 a method is proposed for providing perpendicular magnetic recording media with the object of improving ease of recording without detracting from thermal stability by providing a coupling layer between the two layers of a magnetic recording layer with a two-layer structure, and for improving noise characteristics, S/N characteristics, and other performance aspects, to simultaneously raise the density and improve the ease of recording.
However, if magnetization reversal units are made finer with the aim of further raising the density, the fraction of the magnetic recording layer occupied by the granular structure with a high Ku is increased, the switching magnetic field is increased, and there is a limit to the ability to suppress the decline in ease of recording of the magnetic recording media. And, if the low-Ku layer thickness is increased to ease magnetization reversal with the aim of maintaining ease of recording, the fraction occupied in the high-Ku magnetic layer declines, and higher recording densities become difficult to achieve. Hence, in order to further raise the recording density, a new technology is needed to obtain a high recording resolution while keeping the coercivity Hc, which depends on Ku, at a low level.